The present invention relates to the reduction of sulfur-containing emissions (e.g., SO.sub.2 and SO.sub.3 gases, particulate sulfates, and H.sub.2 SO.sub.4 mist) from a large scale, continuous, flat glass melting operation. The term flat glass refers to glass commercially produced by the float process, plate rolling and grinding, and sheet drawing. Flat glass generally conforms to a relatively narrow composition range as follows:
SiO.sub.2 : 69-75% by weight PA1 Na.sub.2 O: 12-16% by weight PA1 K.sub.2 O: 0-2% by weight PA1 CaO: 8-12% by weight PA1 MgO: 2-5% by weight PA1 Al.sub.2 O.sub.3 : 0-2% by weight PA1 SO.sub.3 : 0.1-0.5% by weight PA1 Fe.sub.2 O.sub.3 : 0-0.7% by weight
Commercial production of flat glass conventionally involves feeding raw glass batch materials into an opening at one end of an elongated melting furnace while withdrawing melted glass through an opening at the opposite end of the furnace and forming it into a continuous flat ribbon. Flat glass batches typically include sand (silica), soda ash (sodium carbonate), limestone (calcium carbonate), dolomite (calcium carbonate and magnesium carbonate), rouge (iron oxide), a source of sulfur such as salt cake, gypsum, slag, etc., and sometimes the raw materials aplite, feldspar, or nepheline syenite. It is also known to use caustic soda in place of soda ash. Minor amounts of additional materials such as colorants may sometimes be used as well, although the present invention relates primarily to glasses in which the sole essential colorant, if any, is iron. These batch ingredients, in finely divided, dry, particulate form, are blended together and usually wetted with water (or caustic soda solution) prior to being introduced into the furnace. Additionally, a substantial amount of cullet (crushed glass) is mixed with the batch ingredients, in amounts usually ranging from about 20% to about 60% of the total glassmaking materials being fed to the furnace.
When introduced to the high temperature conditions within the melting furnace, the raw ingredients undergo chemical reactions and dissolution which, in a continuous flat glass furnace, normally take place within the first half of the furnace or less. The remainder of the furnace is devoted to "fining" (or "refining") and conditioning the glass melt. The process of fining is the removal of gaseous products of reaction from the melt by providing conditions which cause the gas bubbles to rise to the surface and burst or to redissolve in the glass. In a continuous glassmaking operation it is very important that conditions be maintained to enable fining of each portion of the melt to take place within its limited residence time in the fining zone of the furnace. Any gaseous inclusions which are carried out in the product stream form the defects known as "bubbles" (those having diameters larger than 0.25 mm.) or "seeds" (those having diameters smaller than 0.25 mm.) in the glass.
The problem of obtaining adequate fining is especially acute in a flat glassmaking operation since the standards for bubbles and seeds for flat glass are much more stringent than other types of glass such as bottle glass. For example, flat glass having one seed per square foot (0.09 square meter) would be considered rejectable for most flat glass applications, whereas what would be regarded as a very good grade of bottle glass may have on the order of 500 seeds per square foot (0.09 square meter) if formed into a sheet of the same thickness. In order to obtain adequate fining within a reasonable length of furnace, the flat glass industry has heretofore relied on the inclusion of substantial amounts of a sulfur source, usually salt cake (sodium sulfate), together with a carbon source, usually coal, in the batch as fining agents. The salt cake reacts to form substantial volumes of gas, thereby causing gaseous inclusions to grow, which accelerates the movement of bubbles and seeds to the surface of the melt and helps to homogenize the glass. Thus, it has long been the standard practice in the commercial production of flat glass to include substantial amounts of salt cake and coal or their equivalents in the batch ingredients fed to continuous melting furnaces.
Unfortunately, the use of sulfur compounds as fining agents has serious drawbacks. At glass melting temperatures sulfur compounds such as salt cake dissociate or volatilize, resulting in the emission of sulfur-containing gases. These may recombine with water vapor or sodium vapor within the furnace or exhaust passages to form sulfuric acid mist or particulate sodium sulfate, which are not only air pollutants, but have a detrimental effect on the refractory checker-packing in the regenerators of the furnace. Many widely varying proposals for reducing sulfur-containing emissions have been made in the prior art, but none is entirely satisfactory.
One commonly proposed solution is to treat the effluent gas stream to remove the sulfur compounds. However, such as approach is costly and does not reduce the detrimental effects of the emissions on the regenerators. U.S. Pat. Nos. 3,788,832 and 3,880,639 disclose examples of the recovery and recycling of sulfur compounds from the exhaust gas stream by contacting the exhaust gas with incoming batch materials.
Accordingly, attempts have been made to reduce the amount of sulfur compounds used in the melting process as exemplified by U.S. Pat. No. 4,138,235. It would be desirable to reduce sulfurous emissions even further, but reducing the amount of salt cake or other sulfurous fining agents can adversely affect the melting process. Other attempts to reduce or eliminate sulfur from glass melting tanks have utilized alternate fining agents, but none of these has gained acceptance in the flat glass industry due to unfavorable cost and/or effectiveness relative to salt cake and because some undesirably introduce extraneous ions into the product glass. Some proposed substitute fining agents, such as fluorine compounds, present at least as great an emissions problem as sulfur compounds. It has also been proposed that supplemental operations intended to assist melting, such as agglomerating and/or prereacting the batch materials, may result in lower fining agent requirements, but such an additional operation requires substantial capital investment and increased operating costs, and the predicted improvements have not always been obtained in commercial practice.
Practical limits have existed on outright reduction of sulfur being fed to a glass melting furnace because, in order to provide for adequate fining, excess sulfur must be supplied to compensate for volatile losses in early stages of the melting process upstream from where fining takes place. Also, secondary benefits of sulfur to the melting process would suffer from excessive reduction. These secondary melting effects include: enhancement of runoff from the unmelted batch layer, dispersion of sand grains thereby speeding their dissolution, control of foaming, and prevention of "silica scum" on the surface of the pool of molten glass. These effects are primarily liquid phase effects which require the sulfur to be present in the molten glass during early stages of melting. Thus it has been considered important to provide sufficient sulfur source in the batch to provide an ample concentration of sulfur in the melt early in the melting process.
Reduced amounts of salt cake are employed in the manufacture of one type of flat glass: colored glasses which incorporate selenium, cobalt, and nickel oxides, such as those disclosed in U.S. Pat. Nos. 3,296,004 and Re. 25,312. In glasses of this particular type, development of the desired coloration requires that oxidizing conditions be maintained and, therefore, salt cake and coal are partially replaced by oxidizing agents such as sodium nitrate or sodium chloride. The present invention, on the other hand, deals only with glasses which may be categorized as clear, or which contain iron oxide as the essential colorant. Since the oxidizing conditions required in the melting of the selenium, cobalt, and nickel colored glasses are not required for conventional clear and iron tinted glass, the use of such oxidizing agents has generally been limited to the colored specialty glasses.
Other ways of reducing sulfur-containing emissions may be apparent to those of skill in the art, but each has serious drawbacks. For example, volatilization of salt cake may be reduced by lowering the melting furnace temperature, but the output of the furnace would be reduced and completeness of melting may suffer. Another possibility would be to reduce the amount of salt cake employed and compensate by increasing furnace temperatures. But the result would be shorter furnace life and greater fuel consumption. Yet another approach would be to increase the relative amount of cullet charged to the furnace along with the batch materials. This latter approach has been considered by some in the glass industry to be best solution to the emissions problem as evidenced by a report in Business Week, Mar. 31, 1976, pp. 66B, 66H. But reliance on large amounts of cullet is preferably avoided because adequate supplies of suitable cullet are not always available in the flat glass industry, and excessive use of cullet represents inefficient utilization of a flat glass melting furnace in that more fuel is consumed to yield a net amount of product glass. Thus, it would be desirable if sulfur-containing emissions could be reduced without altering the usual temperature conditions in a melting furnace, while at the same time using a relatively high batch-to-cullet ratio.